1. Field of the Invention
The present invention relates to a high strength spongy baked metal composite sheet and to a method for production of the sheet. The high strength spongy baked metal composite sheet is used as a material for producing a filter for high temperature applications, a filter for air cleaners, and an electrode substrate for secondary alkaline batteries. It is particularly useful as a material for the production of an electrode substrate of secondary alkaline batteries.
2. Description of the Background
In general, the electrode substrates of secondary alkaline batteries are prepared from spongy baked metal sheets which have holes which are open on the surfaces of the sheets. These holes are continuous with internal holes (hereinafter referred to as continuous holes), and the sheets thereby have a volume porosity of 70-99%. Furthermore, recently, the electrode substrates of a secondary alkaline batteries have been prepared from spongy baked metal sheets which have a structure including a skeleton part which comprises a baked sintered metal having continuous holes and extremely fine continuous holes in comparison to normal continuous holes (hereinafter referred to as skeleton fine holes). Preferably the continuous holes have an average hole size of 100-700 xcexcm, the skeleton part has an average skeleton fine hole size of 0.5-20 xcexcm and a porosity of 10-55%, and the spongy baked metal sheet as a whole has a porosity of 90-99%.
For the production of the spongy baked metal sheet, in general, the apparatus shown in the cross-sectional diagram thereof in FIG. 12 is used. In FIG. 12, numeral 6 denotes a carrier sheet, 7 a doctor blade, 8 a foaming slurry, 9 a high temperature high humidity vessel, 11 a drying vessel, 12 a hopper, 13 an unwinding reel, 14 a winding reel, and 15, 16 supporting rolls.
The foaming slurry 8, prepared by adding a surfactant and a foaming agent to a slurry made from material powders and a thinner, is stored in hopper 12. As shown in FIG. 12, the foaming slurry 8 is formed as a thin sheet on the carrier sheet 6 by the doctor blade 7. It is foamed to a sponge-like state utilizing the vapor pressure of a volatile organic solvent and the foamability of the surfactant present in the above-mentioned foaming slurry in the high temperature, high humidity vessel 9. It is further dried in the drying vessel 11 so as to produce a spongy green sheet 10xe2x80x2. The spongy green sheet 10xe2x80x2 is degreased and baked in a degreasing device and a baking furnace (not illustrated) so as to produce a spongy baked metal sheet having continuous holes.
In order to strengthen the spongy baked metal sheet which has been produced, in general, the sheet is integrated with a reinforcing layer of a metal layer having ordinary punched holes, a metal mesh, or a plurality of fine metal layers by resistance welding.
However, in order to integrate the spongy baked metal sheet with the metal tape having the punched holes or the metal mesh by resistance welding, at least two steps are required which include the step of preparing the spongy baked metal sheet and the step of resistance welding the spongy baked metal sheet to the metal tape, the metal mesh, or the like. These two steps increase the production costs, and thus it is not preferred method, Moreover, a problem is involved in that at the time of the resistance welding of the spongy baked metal sheet and the metal tape, the metal mesh, or the like, the holes of the spongy baked metal sheet are crushed by an electrode roll employed in the process so that the porosity of the spongy baked metal sheet is lowered.
Another method of producing a high strength spongy baked metal composite sheet involves preparing a thin sheet from a foamed slurry on a metal tape having punched holes or a metal mesh by the doctor blade method, preparing a spongy green sheet having continuous holes by foaming a foaming slurry utilizing the vapor pressure of a volatile organic solvent and the foamability of the surfactant present therein in the constant temperature, high humidity vessel, and degreasing and sintering the spongy green sheet in order to laminate a spongy baked metal sheet having continuous holes on the metal tape having the punched holes or the metal mesh. However, a high strength spongy baked metal composite sheet produced by this method possesses the problem that because the contraction ratio of the spongy green sheet at the time of degreasing and sintering is very large in comparison to the contraction ratio of the metal tape or the metal mesh to which it is bonded with the result that the integrated sheet becomes curved toward the spongy baked metal layer side or cracks are generated in the spongy baked metal layer.
Furthermore, when the integrated sheet is used as an electrode substrate of a secondary alkaline battery, the spongy baked metal layer of a high strength spongy baked metal composite sheet is impregnated with an active material and is rolled with a roll. At that time, the joint which is formed between the spongy baked metal layer and the metal tape may be insufficient thereby causing the spongy baked metal layer to peel from the metal tape.
Accordingly, one object of the present invention is to provide a composite green sheet prepared from a spongy metal layer and a high strength metal reinforcing layer, which does not result in curvature of the sintered product which is prepared.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a high strength, spongy sintered metal composite sheet, comprising a porous, spongy sintered metal layer having continuous holes which are open to the surface of the layer and which are continuous with internal holes of the metal layer, and a high strength, sintered dense metal reinforcing layer having a porosity less than the porosity of the spongy, sintered metal layer, laminated thereon.